Chromized ferrous article



Nov. 21, 1967 G. A. SAMUEL ETAL 3,353,936

CHROMI ZED FERROUS ART I CLE Original Filed Nov. 29, 1962 INVENTORS6320? 2 4.022%

ATTORN EY6 United States Patent 3,353,936 CHROMIZED FERROUS ARTICLEGeorge A. Samuel, Wimereux, Pas-de-Calais, France, and Jerome V. Bell,Wilmington, Del., assignors to Alloy Surfaces Company, Inc., Wilmington,Del., a corporation of Delaware Original application Nov. 29, 1962, Ser.No. 240,858, now Patent No. 3,222,212, dated Dec. 7, 1965. Divided andthis application Sept. 6, 1963, Ser. No. 307,263 The portion of the termof the patent subsequent to Dec. 7, 1982, has been disclaimed 9 Claims.(Cl. 29-196.6)

The present application is a division of our copending application,Serial No. 240,858, filed November 29, 1962, now United States Patent3,222,212. The present application is a continuation-in-part of ourearlier applications, Serial No. 119,085, filed June 23, 1961, SerialNo. 160,764 filed December 20, 1961 and Serial No. 172,231, filedFebruary 9, 1962, all now abandoned. The present application includesclaims to subject matter divided from our prior applications, Serial No.160,764, and No. 172,231 above mentioned.

The present invention relates to a chromized ferrous metal product,particularly low carbon steel and ingot iron, also permissibly medium orhigh carbon steel.

A purpose of the invention is to circulate chromizing gases in auni-directional manner in contact with the surface of ferrous metalwork, which may suitably take the form of an open coil, but may be ofsome other char.- acter, and thus to obtain more rapid and moreeconomical chromizing with lower expenditures of chromium and withgreater throwing power.

A further purpose is to chromize more effectively at low temperature.

A further purpose is to circulate a gaseous atmosphere containing ahalogen gas which essentially remains in the retort, endlessly through asource of chromium not in contact with the work, and then in contactwith the work and then back to the source of chromium, the gaseousatmosphere always flowing continuously in the same direction.

A further purpose is to mount the work in the retort, the work suitablybeing an open coil, so that a stream of chromizing gases which haspassed through a source of chromium not in contact with the work isdirected under positive pressure so that it flows in contact with thework and then returns to the source of chromium without division orshort-circuiting.

A further purpose is to circulate a carrier gas containing ahalogen-containing gas through a source of chromium not in contact withthe work in a chromizing retort, and to maintain the concentration ofhalogen in the halogen-containing gas at a sufiiciently low level as toavoid priming the source of chromium or depositing a large excess ofliquid or solid chromium halide on the source of chromium which wouldtend to react with moisture and interfere with chromizing in a lateroperation.

A further purpose is to utilize a combination of carrier gas andhalogen-containing gas which has an inlet dew point of below 100 F., andpreferably below 110 F., and is substantially free from oxygen.

A further purpose is to control the quantity of oxygen 3,353,936Patented Nov. 21, 1967 "ice (as such or as oxides which can react)present in the retort so that the dew point of the exit gases will bebelow 40 F. or preferably be below 60 F., and will in any case not behigher than +5 F.

A further purpose is to protect the work during heating up to atemperature not exceeding 1100 F. by a nonexplosive gas such as nitrogenand then to eliminate the presence of nitrogen and at highertemperatures avoid nitrogen and thereby avoid nitriding cifects.

A further purpose is to employ a surface area of the source of chromiumto the area of the work which will be between 0.2 to 1 and 10 to 1 orhigher, preferably between 0.8 to 1 and 10 to 1, and most desirablybetween 3to 1 andSto 1.

A further purpose is to conduct the chromizing preferably at lowtemperatures in the range from 1600 to 1800 F., particularly when usingthe system involving a source of chromium, hydrogen as a carrier gas,and hydrogen bromide (or bromine or chromous bromide) as ahalogen-containing gas. Suitable times will be of the order of 5 to 10hours or longer.

A further purpose is to employ the principles of the invention on steelsor other ferrous metal products of low carbon content, having carboncontents below 0.03% and preferably below 0.003

A further purpose is to apply the principles of the invention to steelswhich have been stabilized for example by titanium, columbium, vanadiumor tantalum, so that the carbon content uncombined with the alloyingelement is kept very low, below 0.03% and preferably below 0.003%.

A further purpose is to render the steel more suitable for chromizing bydecarburizing the steel, preferably in the same operation, as byintroducing Wet hydrogen, and in a temperature of the range of 1300 to1400 F. and then thoroughly drying the retort before chromizing.

A further purpose is to obtain a chromized steel which will benon-aging, having an aging index of from 0 to 2%.

A further purpose is to redistribute any superficial skin which isabnormally high in chromium and which may tend to be brittle bydiffusion of the chromium into the base metal by continuing the exposureto hydrogen at or about chromizing temperature while removing thehalogen so that chromizing will cease.

Further purposes appear in the specification and in the claims.

In the drawings we have chosen to illustrate one of the numerous typesof apparatus which may be used in producing the product.

FIGURE 1 is a vertical cross sectional view of one form of apparatus forsubjecting open coils of strip or sheet material to chromizingtreatment.

FIGURE 2 is a horizontal cross sectional view taken substantially on theline 22 of FIGURE 1, and illustrating the form and arrangement of thetrays for carrying the source of chromium.

FIGURE 3 is an enlarged vertical cross sectional view takensubstantially on line 33 of FIGURE 2, of several of the upper trayscarrying the source of chromium.

The particular furnace shown in respect to its constructional detailsforms no part of the present invention.

Describing in illustration but not in limitation and referring to thedrawings: technique'of the present invention it is readily possibleExtensive use has been made in the prior art chromizing processes inwhich a powder pack including a source of chromium such as ferrochromeand inert bodying material are placed in contact with the work and acompound such as ammonium chloride, ammonium bromide, ammoniumbiiiuoride or ammonium iodide is caused to break down liberatinghalogen-containing gas to promote chromizing.

Efforts have also been made in the past to chromize by employing ahalogen-containing gas such as hydrogen chloride, and a carrier gas suchas hydrogen which were passed over a source of chromium to causeformation of chromous chloride and then to pass the chromous chlorideover the work and out of the retort. Both of these processes have beensubject to the limitation that they are relatively slow, cumbersome andexpensive. They also are not always reliable, particularly because ofinfiltration of oxygen and in some cases because of the presence ofmoisture.

In the case where it is desired to produce chromized products from sheetor strip, there are two alternate possibilities. The practice usuallyused has been to Produce the final components and then to chromize themafter forming. This is subject, however, to the disadvantage that thereis a very poor work load in the retort and the parts often suffer fromheat distortion. Also, the effects of previous cold work are destroyed.

On the other hand, it has not been possible in the prior art to obtaineffective chromizing of large areas in coil form.

In the case of the pack method, a serious limitation has been the verypoor heat transfer. Another limitation in the pack method has been thepoor throwing power which made it important to position the work inintimate contact with the pack. Poor finish on the work has also been adifiiculy, as well as poor physical properties of the steel sincetreatment temperatures have been relatively high and also the treatmentshave been rather prolonged.

Where chromizing gas has been generated outside the retort and passedthrough the retort and then out of the retort, in the prior art, thewaste in chemicals has been very serious. Furthermore, the directionalcharacter of the gas flow has produced shadowing effects in thedeposition which limit the throwing power transverse to the direction ofgas flow.

The previous systems have also been very uneconomical from thestandpoint of chemical utilization. The present invention is concernedparticularly with overcoming the difficulties noted and particularlyobtaining a more economical, more efficient and more effectivechromizing operation.

In producing the products of the invention, the chromizing gases, whichwill preferably consist of a carrier gas plus a halogen-containing gas,are sequentially and repetitively passed through or over a source ofchromium such as metallic chromium or ferrochrome, and then in contactwith the work, and then back through or over the source of chromium toregenerate and again in contact with the work. This is accomplishedunder forced circulation as from a pump (or fan), so that the entiregaseous content of the retort can be recycled in a time as short as onesecond.

This process, therefore, permits very efiicient charging of the retort,since it is no longer necessary to place in contact with, or interposebetween, the many layers of ferrous metal work, a powder pack as in thewell known powder pack prior processes. The throwing power of the retortgaseous content is so great that it is possible to effectively chromizerelatively tremendous areas of work as for example coiled sheet or stripin which the individual laps of the coils are opened sufficiently toallow gas circulation through the coil. The best prior art. practice bythe pack method in chromizing sheets has utilized a spacing of threesheets to the inch. By the.

to chromize with laps of coil which are as close as 4 to 5 to the inch.

Since there is no longer any need for refractory materials such asalumina, the time of heating up the retort can be reduced to a minimum.Furthermore, the efficiency of charging the retort is greatly increasedbecause no extra space must be occupied by the powder ack.

p As compared with the prior process in which chromizing gases have beengenerated outside the retort, the process for producing the presentproducts uses the same gas content over and over, instead of dischargingit from the retort when it has once come in contact with the work. As aconsequence, it is only necessary to introduce or withdraw gas from theretort once the chromizing operation has started in sufficient quantityto maintain a superatmospheric pressure in the retort or to compensatefor infiltration of air and to otherwise remove objectionable materialsuch as water vapor which may have been present in the retort or mayhave been formed due to reduction of oxides present. Therefore, thechemical consumption is reduced to a minimum.

By reusing the gas, the waste of chromium and other metal halides, whichwould result from wasting the gas when it has once been used forchromizing is avoided, and also the halogen itself is conserved.

Use has been made in the prior art of static gaseous processes, in whichthe gaseous content which is to accomplish chromizing is essentially allcontained within a sealed retort, in some cases with venting provided.These processes are extremely slow, and as compared with them, thethrowing power in making the present products is infinite since thegases can be made to follow any desired course within the rotort whichwill bring them in contact with all parts of the work.

Efforts have been made in the past to chromize using a rotating retortcontaining gases and a powdered or granulated pack. In this case,however, the gas at any point is a haphazard mixture of regenerated andunre generated gas unlike the gaseous content of the retort in makingthe products of the present invention.

The products of the present invention are often large, for example ofopen coil steel sheet, since there is no appreciable limitation ofsoaking time and no serious problem of heat transfer. Single coils aslarge as 20,000 pounds may be chromized in a single retort operation andin a time of the order of only 10 hours at chnomizing temperature.

From the standpoint of the product, there have been serious difficultiesin chromizing by prior art practices. When using the pack method, undercertain circumstances it is likely that chromium from the source ofchromium will sinter on the surface and produce roughness. There hadalso been difliculty through producing relatively abnormally lightchromizing at certain places where gaseous diffusion or where packmaterial were not present.

The present invention lends itself particularly to the production ofvery high quality chromized steel products since chromizing is carriedout in the absence of nitrogen and the chromizing operation can relievefrom the presence of dissolved nitrogen. The absence of nitrogen is alsoadvantageous because it prevents nitriding the source of chromium whichdeteriorates the source of chromium and also tends to contaminate thesteel.

The invention is particularly suitable for chromizing at lowtemperatures, which tends to minimize or avoid harm to the steel, eitherby grain growth or due to distortion. The chromized case which isobtained in the present invention is of very superior quality from thestandpoint both of ductility and corrosion resistance because of theextremely low carbon content of the ferrous metal work, resulting in lowchromium carbide content in the chromized layer. The carbon content ofthe case may be as low as 0.02% carbon when the case is formed onZO-gage steel which has been decarburized prior to chromizing.

Many prior art processes which have used a pack have depended uponpriming of the pack so that it is impregnated with solid or liquid phasechromous chloride. It has been found that the priming of the source ofchromium is likely to cause serious difiiculties in subsequentprocessing. Certain processes require priming as a separate technique,which is time consuming and expensive, and use the priming material asthe source of chromizing potential until it is so depleted that it mustbe replaced. For example, in some instances in the prior art extremecare must be exercised in protecting the primed source of chromium frommoisture and oxidation, as otherwise this will lead to very high dewpoints in the chromizing gases, impairing the quality of the chromizing.

GASEOUS CONTENT While the products of the present invention can beproduced effectively using any one of the halogens, as for examplehydrogen chloride, chromous chloride, hydrogen fluoride, chromousfluoride, chlorine, hydrogen bromide, bromine, chromous bromide,hydrogen iodide, chromous iodide and various other metal halides havingsufficiently high vapor pressure at chromizing temperature, such asaluminum chloride and aluminum bromide, there are great advantages inusing hydrogen bromide, chromous bromide, or bromine, as laterexplained, and particularly when operating at temperatures of 1600 to1800 F.

There is also an important advantage in using hydrogen as a carrier gaswhen a bromine chromizing system is being employed, that is, hydrogenplus hydrogen bromide, or hydrogen plus bromine, or hydrogen pluschromous bromide. Hydrogen is readily available, can be obtained withvery low oxygen contents and nitrogen contents and is not objectionablefrom the standpoint of effect on the steel. Hydrogen tends to promotethe reduction type of chromizing reaction as follows:

Thus, the hydrogen carrier gas acts as a conveyor of thehalogen-containing gas, and tends to reduce oxygen which may be presentin the retort for example as oxide. Hydrogen procured with an oxygencontent less than 2 p.p.m. and a dew point less than l F., and anitrogen content less than 2 p.p.m. is obtainable at relatively lowcost, and can be used in copious quantities to flush out the system andbe added during chromizing in such quantities as are required to removeany oxygen introduced by leakage and also to remove water formed eitherby reaction with oxides which are present, or water driven off from theinterior of the retort which was formerly absorbed or chemicallycombined. It will be understood that in many cases a porous source ofchromium will be used which may give off moisture, oxygen or otherobjectionable materials under chromizing conditions. The source ofchromium likely will contain some chromium oxide, and the hydrogencarrier gas will reduce this to metallic chromium when supplied insufficient quantities, and thus maintain low dew points in the exitgases as later explained.

It will of course be evident that adequate inspection techniques shouldbe instituted to insure the purity of the hydrogen, as this is acritical feature.

A convenient way to determine whether there is suflicient freedom frommoisture vapor and oxide in the retort is to determine the dew point ofexit gases. Dew points as high as +5 F. can be tolerated in the exitgases, although for best results the dew points in the exit gases shouldbe lower than 40 or preferably lower than -60 F.

In a retort having an internal free space of approximately 300 cubicfeet, it was desirable to maintain a flow of hydrogen carrier gas of theorder of 2000 cubic feet per hour during the drying period prior tomixing with the halogen-containing gas, and after chromizing started andhalogen-containing gas was added, it was found that a flow rate of 300cubic feet per hour was adequate (input of hydrogen at standardconditions). These gases are measured at 70 F. and one atmosphere.

In order to do effective chromizing with hydrogen bromide or chemicallyequivalent amount of chromous bromide or bromine in a carrier gas ofhydrogen, it is quite important that a minimum concentration of hydrogenbromide by volume of 2.00% be maintained in the tempera ture range from1600 to 1800 F. where bromide chromizing is most effective. If theconcentration of hydrogen bromide or equivalent is less than 2.00% byvolume, satisfactory chromizing is not obtained.

In many cases, it is also important that a maximum percentage ofhydrogen bromide or a chemically equivalent percentage of chromousbromide or bromine be maintained in the mixture with hydrogen carriergaswhich is sufiiciently low to prevent the deposition in the retort ofsolid or liquid chromous bromide in any substantial quantity. If solidor liquid chromous bromide deposits, difficulty is then likely to beencountered by absorption of oxygen and moisture when the retort isopened, and a great detal of delay and waste of hydrogen will occur instarting up the next heat before adequately low dew points can beobtained, in the retort atmosphere and effective chromizing can start.

While the presence of deposited solid or liquid chromous bromide on thesurface of the retort or on the surface of massive (dense) chromium orferrochrome is objectionable, for the reasons stated and also because ofthe tendency to clog up gas passages, traps, and the like, thedifficulty is much more serious when porous ferrochrome is used, forexample, ferrochrome produced by powder metallurgy techniques includingvacuum sintering. In this case the solid, or more particularly liquidchromous bromide, tends to impregnate the porous ferrochrome and isparticularly diflicult to free from oxygen and moisture in subsequentcycles, acting as a continual drag on the chromizing potential of theretort.

It will, of course, be evident that if minor amounts of solid or liquidchromous bromide are deposited on the surface of the retort, they can beremoved by washing or dissolving in water, but this procedure is noteffective with porous ferrochrome.

In the preferred procedure, sufficient chromous bromide is formed tojust saturate the gas phase inside the retort. From this standpoint theconcentration of hydrogen bromide or the equivalent should be as closeas convenient to but not substantially exceeding the upper limits set.

The following table shows maximum and minimum limits for hydrogenbromide in percentages by volume measured at 70 F. and one atmospherefor various temperatures in the range from 1600 to 1800 F. This appliesonly to mixtures of hydrogen bromide in hydrogen, although chemicallyequivalent percentages will likewise apply for mixtures of chromousbromide with hydrogen and of bromine with hydrogen.

It will be evident that the quantities of gases can be determined byweighing the gas cylinders from which the gases are supplied.

Where bromine is used instead of hydrogen bromide, with the hydrogencarrier gas, it is desirable to bubble the hydrogen through the bromineor pass the hydrogen through a chamber in which the bromide is beingevaporated. However, this procedure is not recommended in case theprocess of the invention is used with chlorine because of the extremehazard of an explosion when chlorine is introduced into hydrogen.

It will be evident that mixtures of halogen-containing gases whichcontain different halides may be employed in the process of theinvention if desired, such as hydrogen bromide plus hydrogen chloride;hydrogen bromide plus hydrogen iodide; hydrogen iodide plus hydrogenchloride; hydrogen fiuoride plus hydrogen bromide; hydrogen fluorideplus hydrogen chloride; hydrogen fiuoride plus hydrogen iodide, etc.

SEQUENTIAL OPERATION It is of great importance in some aspects of thetechnique for making the present products to provide a positive means ofcirculating the gases in the retort, along with passages or chamberswhich will secure a sequential flow of substantially all the gases inthe retort.

The positive circulation of the gases will conveniently be applied byany suitable pumping means, such as a fan or blower. Experience hasindicated that for best re sults the pump should be capable ofrecirculating substantially all the gases of the retort at a high rate,conveniently once each second. In order to achieve this result, it hasbeen found that in a retort having a free volume of the order of 300cubic feet, a circulation flow of 13,600 cubic feet per minute (measuredat 70 F. and one atmosphere pressure) is satisfactory.

It is quite important that the direction of gas flow will cause thegaseous atmosphere to pass through or over the source of chromium, so asto regenerate the gases and form new chromous bromide or other chromoushalide, and then to pass the gases after regeneration over or around thework so as to accomplish chromizing. The gases should then be returnedas promptly as possible to the intake of the pump or fan in order tocome in contact again with the source of chromium. It is important forthe success of the reaction that both the source of chromium and alsothe work be held at chromizing temperature and that the gases be atchromizing temperature, so that if the regeneration is accomplished bypassing the gases over the source of chromium in an adjoining orseparate retort chamber, such chamber should be adequately maintained ata temperature of approximately the chromizing temperature. It will beevident that once a chromizing cycle is completed, the source ofchromium will be depleted in chromium content at the surface. It is verydesirable prior to the next cycle or by the beginning of the next cycleto promote surface chromium restoration by diffusion from the interiorof the individual particle to build up the chromium concentration at thesurface of the source of chromium.

This is accomplished by allowing a period of several hours to elapse inthe next chromizing cycle during which the source of chromium is held atan elevated temperature, preferably equal to or greater than thechromizing temperature, prior to the introduction of halogen and theinception of chromizing.

In the preferred apparatus, the source of chromium is located adjacentthe heating means, and is therefore, at a slightly higher temperaturethan the work. This promotes more rapid chromium restoration in theinitial stages of the new cycle prior to the introduction of halogen,and also allows the chromium source to supply chromium to thehalogen-containing gases more rapidly during the chromizing cycle, Allconnecting passages must also be maintained at chromizing temperature toavoid condensation of the chromium halides.

The pump, of course, can be anywhere in the system.

SOURCE OF CHROMIUM The source of chromium for use in making the productsof the invention can either be chromium or an alloy high in chromiumsuch as ferrochrome. Where ferrochroms is used, it should be the lowcarbon and low nitrogen grades so as not to deposit nitrogen or carbon,and should preferably have a chromium content in excess of 65%, atypical analysis by weight being as follows:

Chromium 70% maximum, Silicon 2% maximum. Carbon 0.015% maximum.Nitrogen 0.025% maximum. Iron Substantially balance,

The ferrochrome can be of the powder metallurgy porous type of the densesolid type if desired. Chromium can be used. A typical analysis is:

Chromium 98.5% minimum. Carbon 0.015% maximum. Nitrogen 0.025% maximum.

It has been found that higher superficial chromium contents in the casemay be obtained when chromium met-a1 is used as source of chromiumrather than ferrochrome. Thus, in a typical example, superficialchromium content in the case of 44% was obtained using chromium metalwhereas when undepleted ferrochrome was used, the superficial chromiumcontent of the case was 42%. In both instances a hydrogenbromide-hydrogen system was used at a temperature of 1650 F., and thechromizing was carried on at chromizing temperature.

While various sizes of chromium or ferrochrome particles can be used, inmost cases it will be desirable to employ particles larger than 60 mesh(Tyler standard mesh per linear inch) and not in excess of A inch. Acommon nominal size is inch or inch. The size is chosen in order tostrike a reasonable balance between the surface area of the source ofchromium which should desirably be large and the case with which thegases can penetrate the source of chromium.

Ferrochrome particles having the following sizes were studied in detailand actual counts of the particles per pound were made:

TABLE II Size in inches No. of granules per pound Aa 16 (10 70 (10 %i,57 (10 /16, +l'2 67 /1e, /64 19 /2, /e4 39 The particles were generallycubical or rectangular. Accordingly, it was possible to calculate thesurface area in square feet per pound of ferrochrome particles and thefollowing data were obtained:

TABLE III I Surface area in Size in inches: square feet per pound %1,2.3 /8, +J/1G 2.6 /8, 4.4 /te, +%2 /1c, +%4 /32, +%i 3- The surface areaof the source of chromium should bear a relationship to the surface areaof the work, which will be in the range of 0.2 to 1 to 10 to 1, andpreferably 0.8 to 1 to 10 to 1, and most desirably between 3 to 1 and 5to 1. For best results the ratio should be about 4 to 1. Largerproportions of surface area of the source of chromium may be employed,but are not necessary in common practice. Smaller area ratios decreasethe chromium content of the case.

The area ratios have an etfect on the superfiicial chromium percentageand the average chromium percentage in the case. Using treatments at1650 F. for hours at heat, with between 3.2 and 4.2% by volume ofhydrogen bromide in hydrogen and a steel of A181 1010 composition whichwas decarburized before chromizing to a carbon content of 0.002%, thevalues shown in the following table were obtained:

While chromizing to produce the products of the present invention can becarried out at temperatures in the range between 1500 and 2300 F., thepreferred temperature range should be between 1600 and 1800 F. andpreferably between 1600 and 1750" F. Where the hydrogen bromide-hydrogensystem is employed. Chromous bromide, the efiective chromizing agent,has an adequate partial vapor pressure within this temperature range.

TABLE V Vapor pressure of chromous bromide in atmosphere Temperature, F.Atmospheres 1600 0.041 1800 0.19

It will be evident that if a lower rate of transportation of gases isemployed, it may be desirable to use a system having higher vaporpressures such as the chromous iodide system in order to get aseffective chromizing under the new conditions.

The preferred temperature range in using the hydrogen bromide-hydrogensystem is 1675 to 1700 P. where open coils are being treated in therange of gages from 24 through 16 for thickness of the sheet. Using thistemperature and a time not exceeding 10 hours chromiz/ed case depths of0.0015 inch minimum can readily be obtained at 1700 F., and after hoursat 1660 F. case depths of 0.0011 inch can readily be obtained.

For low carbon steels having an uncombined carbon content not exceeding0.03% where the effective chromizing agent is chromous bromide, it ispreferred ordinarily not to go to temperatures above 1750 F. Where thecarbon content is greater than 0.08% temperatures no higher than 1900 F.are preferred, but in such cases chromous chloride or chromous fluoridewill preferably be used as the effective chromizing agent.

Steel composition The most important aspect of the composition of thesteel or other ferrous metal is its carbon content. It is important inobtainingductile cases on open coil strip and sheet to have carboncontents not exceeding 0.03% and preferably not exceeding 0.003%.

Steels and ingot irons of such low carbon contents are availablecommercially.

In many cases, however, it is preferable to obtain the low carboncontent by starting with an initial moderately low carbon content suchas A181 1010 steel, and then further reducing the carbon content bydecarburizing. The decarburizing can be carried out as a separateoperation using any well recognized decarburizing technique. It is,however, preferable to decarburize prior to the chromizing cycle but inthe same heat. This can be accomplished rather readily by subjecting theopen coil or other ferrous metal work to wet hydrogen at a temperaturein the range of 1300 to 1400 F. An appreciable quantity of moisture ispresent in the hydrogen. An important aspect of the present invention isthat the source of chromium is not primed and therefore it is notnecessary to eliminate the effect of this moisture on primed chromium orferrochrome prior to chromizing. After decarburization is complete,reducing the carbon content to a level below 0.003 and suitably of theorder of 0.002%, it is merely necessary to pass dry hydrogen through thesystem during the heat up period to chromizing temperature in order toobtain adequately low dew points on the exit gases in order to startchromizing.

It will be understood that where decarburization is carried out in thepresence of ferrochrome or chrome using wet hydrogen at say 1300 to 1400F., the source of chromium itself is oxidized and to some extentcarburized. However, after the steel has given up substantially all ofits carbon in the form of efiluent carbon monoxide gas and even afterthe carbon monoxide concentration of the efiluent gases has reached alow value of 0.01% by volume, the use of wet hydrogen is continued forseveral hours in order to reduce the carbon content of the chromiumsource to its original value. Subsequent introduction of dry hydrogenincident to the elevation of temperature for chromizing, results indeoxidizing of the source of chromium, so that effective chromizing canbe carried out at chromizing temperature.

It sometimes is desirable at the beginning of chromizing to introduce aricher halogen content than Will subsequently be used. This acceleratesthe formation of the proper amount of chromous halide especially whenvery low flow rates of carrier gas are employed. Thus it may beadvantageous in this case to use an initial content of halogen in thehydrogen stream of say ten to twelve percent of hydrogen bromide, andafter a short time, say one hour, reduce the halogen content to thelimits previously referred to. In some cases the steel may be stabilizedby the alloymg of titanium, columbium, vanadium or tantalum in aquantity of the order of at least four times the carbon content. Suchsteels should not have an uncombined carbon content in excess of 0.03%,and preferably not in excess of 0.003%.

The invention is also applicable to silicon relay steels which oftencontain silicon in the range from 1 to 4 /2 with adequately low carboncontents as specified above.

The product of the invention need not have chromium depletion at thesurface, for example in straight chromium and chromium nickel steels ofstainless type including the 200, 300, 400 and 500 series of stainlesssteel. Thus, annealing and other heat treating operations can be carriedout without special precautions against chormium depletion and thosestainless steels which have been annealed in non-protective atmospherescan then be treated in the process described herein so as to bring aboutappropriate chromium restoration at the surface of the stainless steel.

There is evidence from immersion testing in corrosive media such as 5%aerated salt water solution at room temperature that for most favorablecorrosion resistance the carbon content in the chromized case should bebelow 0.05 Thus where a specimen with a carbon content in the chromizedcase of 0.32% failed after 43 hours, specimens with carbon contents inthe chromized case of 0.061%, 0.094% and 0.051% withstood 120, 144 and168 hours salt immersion respectively. Yet the chromium content at thesurface of the failed specimen was 30% and the chromium contents at thesurfaces of the good specimens were 33%, 28% and 33% respectively. Otherdata indicate that reasonably good salt spray resistance can be obtainedif the carbon content is limited to 0.15% and for outstanding resistanceto salt immersion or salt spray the carbon content in the chromized caseshould be kept below 0.05%.

HEATING AND COOLING In actually producing the product of the invention,it may as a matter of economy or convenience be desirable to accomplishlower temperature heating and lower temperature cooling in an atmosphereother than an atmosphere of pure hydrogen. Thus, for example in heatingup to a temperature of 1100 F. the atmosphere, if desired, may benitrogen or nitrogen plus hydrogen in any desired proportions butgenerally less than 8% hydrogen by volume in order to avoid explosionproblems. Similarly, at the end of the cycle the work may be cooled downfrom 1100 F. to room temperature in such an atmosphere of nitrogen ornitrogen plus hydrogen, without difficulty. The nitrogen at these lowtemperatures is not objectionable from the standpoint of reaction withthe work to cause poor aging properties.

COIL SPACING The spacing between adjoining surfaces of one lap and thenext lap of the coil should be adequate to permit proper gas flow in theopen coil. Good results have been obtained using such spacing in theorder of 0.13 to 0.22 inch. This spacing may be maintained in a mannerwhich has been previously practiced in the annealing art, by usingspacers such as twistover wires at the edges of the coil which haveintruding loops at the edges of turns. Also, it will be evident thatspacing can be obtained by corrugated strips or the like applied betweenlaps at the top and bottom edges of the coil.

DIFFUSION In some cases there is a tendency to deposit localized areashaving superficial chromium skins which are brittle and lacking incorrosion resistance. In order to overcome this difficulty, it is inmany cases desirable to remove the halogen gas and hold the work at orabout chromizing temperature for an additional time to permitrediifusion of this skin. A time of 1 hour at chromizing temperature indry hydrogen is suhicient. The flow of hydrogen during the redifiusionperiod need merely be sufficient to maintain a dew point of less than F.and preferably less than 40 F., or most desirably less than 60 F. Thehydrogen flow should be continued in adequate quantities to prevent thedanger of building up an explosive mixture with air in the retort.

PROPERTIES OF THE CHROMIZED WORK After treatment times of 10 hours at1650 F. using the hydrogen bromide and hydrogen system, average casedepths of 0.0010 inch have been obtained with an average chromiumcontent of at least and as high as 23%, and a surface chromium contentof at least 31% and as high as 34%.

One of the important properties is that the case is quite ductile, sothat the sheet is capable of undergoing an 1?. Olsen cupping test of0.40 inch without failure of the core. The case started to orange peelat a value of 0.25 inch. The steel is free from aging and has an agingindex of between 0 and 2%.

Thus, the steel of the invention has a number of ex-- ceptionalproperties which cooperate to provide good. formability. An ASTM grainsize of the core of the: chromized steel not coarser than 3 is readilyobtained. A carbon content in the core can readily be obtained whichdoes not exceed 0.0015 and a nitrogen content. in the core can readilybe obtained which does not exceed 0.0005%. As already explained, thesteel has an aging index between 0 and 2%. The steel can readily beobtained with an Olsen cupping value of the core of at least 0.40, andan Olsen cupping value of the case at which orange peel begins to appearof at least 0.25 inch.

One important aspect of the invention is that the carbon and nitrogencontents of the case will be related to the gage of the steel which hasbeen chromized. Starting with a decarburized plain carbon steel whichhas a carbon content not in excess of 0.002%, and chromizing on bothsides, the maximum carbon content in the chromized case will be asfollows in relation to sheet thickness:

TABLE VI Maximum Percent Carbon in Case on Sheet chromized on Thickness(in.) Both Sides Typical physical properties for the chromized plaincarbon steel with a carbon content of less than 0.0015 is tensilestrength 37,000 p.s.i.; yield strength 17,100 p.s.i.; elongation in 2inches 45%. The ASTM grain size ranges from 3 to 4.

Using titanium stabilized steel (0.05% carbon, 0.25% titanium) andchromizing at 1650 F. for 15 hours at heat, it was found thatexceptionally good mechanical properties accompanied with fine grainsize were obtained. The chromizing was of normal high quality. Theterminal grain size was ASTM No. 6-8, while the yield strength was21,000 p.s.i. and the ultimate tensile strength was 46,000 p.s.i. andthe elongation in 2 inches was 38%.

Table VII shows the results of chromizing according to the invention,but in several cases deliberately changing variables to illustrate theeffect of various factors. In some cases where proper control has notbeen obtained, poorer chromizing has resulted.

In all cases the steel was a 20 gage coil of the weight and carboncontent shown, the steel being plain carbon.

The weight of low carbon ferrochrome is indicated in the variousexamples. The size of the ferrochrome particles was in every case belowA inch and larger than %4 inch.

The chromizing temperature in every case was 1650 F. to 1660 F. and thechromizing times are given.

TABLE VII Heat Number 841 842 864 870 954 896 897 898 902 1004 1023 1080Coil weight (lbs) 3, 000 2, 700 2, 900 3, 000 2, 200 3, 100 3, 100 3,100 3, 100 1, 800 3, 600 2, 940 Percent carbon content of steel prior tochromizing 0.002 0. 002 0.001 0.001 0. 001 0.002 0. 002 0. 002 0. 002.001 Note 002 Spacing between laps (in.) 0. L 0.20 0. 20 0.18 0.18 0.180.18 0.18 0.18 0. 20 0.15 23 Weight of ferrochrome (lbs) 3, 600 3, 5903, 542 3, 442 3, 480 3, 600 1, 800 1, 655 815 4, 380 3, 860 2, 160 Sizeof ferrochrolne n.) %+l2 }is+l-64 Ratio of area of terrochrome, area ofsteel 2. 1 2. 3 2. 0 2. 0 2. 7 2. 0 1.0 0.92 0. 45 4. 5 3. 6 3.9 Exitdew point of hydrogen just prior to use of hydrogen halide F.) -95 72 64+3 30 62 80 65 -59 42 -20 41 Percent by volume of hydrogen halideused 1. 45 3. 64 5 7. 82 3. 68 4.01 3. 95 3. 75 3. 16 11. 4 9.0 4. 4 HBrHBr HBr HBr H01 HBr HBr HBr HBr HBr HBr HBr Chromizing time (hrs) 9. 510.0 5.0 10.0 10.0 7.0 10.0 12.0 15 15 Case depth (mils) 1.10 1.10 0.900. 50 1.0 0.80 0.77 0.75 1.05 1.13 1.01 Average percent chromium in cas12 20.0 22. 9 22. 5 12 20. 5 19.3 18. 2 16.6 22 23 Percent chromium atthe surface of the case 12 26 31 12-18 28 28 23 18 32 29 27 Aging index(percent) 15 1.5 0 0-2-8 2.0-16.5 0 0 0 0 0 0 *NOTE.Coi1 decarburized insame heat, but as initial low temperature phase prior to chromizingphase.

During chromizing in each instance a hydrogen flow of 300 to 1000 cubicfeet per hour measured at 70 F. and one atmosphere pressure was providedso as to excludc infiltration of oxygen and moisture, and permitmeasurement of exit dew point as indicated.

In Heat 841, with a concentration of hydrogen bromide in hydrogen of1.45% by volume as measured at 70 F. and one atmosphere, poor chromizingwas obtained, there being too little hydrogen bromide present.

When the concentration of hydrogen bromide as measurcd above wasincreased to 3.65% by volume as in Heat 842, good chromizing wasproduced. It will be noted that the steel has an aging index of 1.5%,whereas the aging index was 15% in Heat 841, this being the normal agingindex for this steel unchromized.

In Heat 864 where the hydrogen bromide concentration as measured abovewas 7.05% by volume, it is shown that there is very slight improvementin the chromizing, although the aging index has been reduced to 0%. Inthis case there was a deposit of a great excess of chromous bromide. Theporous ferrochrome used in this heat showed a chromous bromide contentof 245 grams per 100 pounds. This indicates a substantial level ofpriming.

It is interesting to note that in Heat 842 where the hydrogen bromideconcentration was 3.64% by volume, the porous ferrochrome was found tocontain no chromous bromide.

In Heat 870, the results obtained are very similar to those of Heats 842and 864, despite the fact that the exit gases now have a high dew pointof +3 F. at the beginning of the chromizing cycle, and chromizing wasaccomplished in the relatively short time of five hours, although theconcentration of hydrogen bromide was increased to 7.82% by volume, asmeasured above.

In order to compare the benefit obtained by chromous bromide with thechromizing Where chloride was used at this low temperature of 1650 F.,Heat 954 employs a concentration of hydrogen chloride of 3.68% by volumemeasured at 70 F. and one atmosphere, which is substantially comparablewith the concentration of hydrogen bromide in Heat 842. However, in Heat954 poor chromizing was obtained, as indicated by the low case depth of0.50 mi the low average chromium concentration in the case of less than12%, the low surface concentration of chromium in the case of 12 to 18%,and the wide variation in aging index between the outside of the coiland the inside (2% to 16.5%). This is a clear indication of poorchromizing because the vapor pressure of chromous chloride is too low atthe temperature of 1650 F. to provide an effective chromizing in theparticular equipment at the circulation rate obtained. Notwithstandingthe poor chromizing, there was actually priming of the ferrochrome bychromous chloride to the extent of 216 grams per 100 pounds offerrochrome.

Heats 896, 897, 898 and 902 employ conditions which are the same exceptthat the ratio of the area of the source of chromium to the area of thesteel progressively decreases from a maximum of 2.0 in the case of Heat896 to a minimum of 0.45 in the case of Heat 902. Even in the case ofHeat 896 the average chromium content in the case and the percentage ofchromium at the surface of the case are not high enough to indicateoptimum chromizing, although they were adequate from the standpoint ofproducing a low aging index in all cases, and the product issatisfactory for many applications. The important point to note,however, is that the average chromium content in the case decreases asthe ratio of the area of the source of chromium to the area of the steeldecreases, and the same is true of the percentage chromium at thesurface of the case.

In Heat 1023 the 3600 pound AISI 1008 coil was decarburized at 1350 F.using inlet hydrogen at F. dew point, and in the same cycle, but at ahigher temperature after dry out with -1l0 F. dew point hydrogen it waschromized using more finely crushed fcrrochromium in the size range.

In Heat 1080 fine enough ferrochromium was used so that the weight ofthe 20 gauge coil (2940 lbs.) now exceeds the weight of fcrrochromium(2160 lbs.).

APPARATUS FOR MAKING THE PRODUCT While anyone of a wide variety ofapparatus can be used to make the product, 1 illustrate in the drawing avery simple and convenient form which in itself is not part of thepresent invention. This apparatus was used in the above examples.

The apparatus may be in some way similar to that described by J. Arnoldin Iron and Steel Engineer, August 1960, pages 91 to 111, whichdiscusses open coil annealing; patent application Serial No. 155, 585,filed November 29, 1961, for Apparatus for Surface Coating of StripMetal or the Like, now United States Patent 3,183,888.

Referring now to FIGURES l, 2 and 3, the chromizing apparatusillustrated includes a base structure 20 in which is supported a chargesupport and a diffuser 21 having a bottom wall 22, a top wall 23, and aplurality of radially extending diffusing vanes 24. A centrifugal typefan or blower 25 is supported in the central portion of diffuser 21 on ashaft 26 and is adapted to be driven from a suitable power source (notshown) through a drive pulley 27 or the like. The top wall 23 ofdiffuser 21 is provided with a central opening 28 forming the inlet tothe blower 25 and the outer periphery of diffuser 21 is open at 30forming an annular outlet passage for the atmosphere moved outwardly bythe blower 25.

Supported on the upper wall portion '23 of the diifuscr 21 is a plenumchamber structure 31 having an outer wall 32, and inclined annularbaffle wall 33 and a plurality of radially extending support wcbs 34.Carried on these radially support webs 34 is a perforate coil supporting:grid 35, and outer imperforate ring member 36 and an imperforate centerclosure plate 37.

When it is desired to chromize a coil of strip steel, the coil is firstrewound into open form with the laps of the coil spaced apart preferablya distance equal to from one to ten times the thickness of the stripbeing treated. This may conveniently be effected by the proceduredescribed in the application of Lee Wilson and Edwin A. Corns, SerialNo. 639,939, filed February 13, 1957, now United States Patent3,114,539. Sue-h an open coil is indicated at 38 in FIGURE 1 and iscarried by the grid of the plenum chamber 31 after being placed thereonby an electrornagnet or other suitable lifting means. As seen in FIGURE1, the coil 38 is disposed with its axis vertical and with the outer lapjust overlying the imperforate outer ring 36 of the plenum chamber 31and with its inner lap just overlying the outer edge of the centerclosure plate 37.

1 Thus, it will be evident that the fan can suck the chromizing gasesthrough the open coil in an axial direction without the possibility ofshort-circuiting to bring through the coil, gases which have not beenregenerated as later described.

A series of superimposed annular material trays 40 for the source ofchromium are arranged vertically spaced apart, in stacked or tieredrelation, as best seen in FIG- URE 3. These trays 40 are supported andheld in position by inclined annular baffle spaces 41 and verticallyextending circumferentially spaced apart spacer rods 42. Each tray 40has an annular inner wall 42' and outer wall 43, is open at the top, andhas a perforate bottom wall 44 which may conveniently be made ofexpanded metal or the like having suflicient strength and rigidity toserve its structural function while permitting the free flow ofatmosphere therethrough. On top of the perforate bottom wall 44 -is alayer of wire screening 45 of sufhciently fine mesh to prevent thepassage of the source of chromium, indicated at 46 in the drawings,therethrough. Another annular ring or layer of screening 47 is placed ontop of the source of chromium on the trays 40 to prevent it being pickedup by the atmosphere moving through the trays and carried into the coil38, particularly when the bed is fluidized.

As seen in FIGURE 1, the stack of trays 40 is supported on the upperwall portion 23 of the charge support and diffuser 21 just outside ofthe plenum chamber 31 and extends up in spaced relation to the outerperiphery of the coil 38. To facilitate handling, it is preferable thatthe top tray 40 of the assembly be disposed slightly below the top ofthe coil 38 so that these trays will not have to be removed each time anew coil 38 is positioned for processing.

After the open coil 33 is placed on the grid 35 of plenum chamber 31within the bank of trays 40 as seen in FIGURE 1, a removable housing inthe form of an inner cover 48 having an open bottom bell shape is placedover the trays 40 and coil 38. This housing or inner cover 48 issupported at its lower edge on the base 2%} and has an outwardlyextending portion 50 having dependent flanges 51 and '52 which fit intoa sealing channel 53 in the base structure 20. This channel 53 containsoil, sand or other suitable sealing material to form a gas-tight sealbetween the base 20 and the inner cover 48. If desired, :a water cooledgasket can be used.

To provide the necessary heat to accomplish the chromizing action abell-type furnace structure 54 is remov- :ably supported on the basestructure 20 and carries a series of circumferentially arranged heatingelements such as the radiant combustion tubes 55 of well known type. Theheat from the combustion tubes 55 is transferred through and by the wallof the inner cover 48 to the atmosphere circulating within the innercover. It will be understood that although combustion tubes areillustrated as .a source of heat, electrical heating elements or othersuit- .able sources .of .heat may be employed. It "will also .be

understood that the base 20, diffuser 21, plenum chamber 31, trays 40,inner cover 48 and furnace 54 are preferably circular in horizontalcross-sectional form and that the furnace structure 54 is placed overthe inner cover 48 preparatory to a chromizing operation and is removedafter the heating operation is completed.

In order to direct the flow of atmosphere within the inner cover 48 inthe desired path and to cause substantially all of the circulatingatmosphere to pass repeatedly through the spaces between the laps of thecoil without lay-passing or channeling around the outside or through theinside of the coil, we provide a removable top baffle member 56 whichhas a downwardly depending outer flange portion 57 adapted to rest in anannular sealing trough or channel 58 disposed at the outer periphery ofthe uppermost tray 40. Suitable granular sealing material may be carriedin the trough 58 to form a substantially gas-tight seal between the toptray 46 and the batlle member '56. This baffie member 56 is removablysupported in the channel 58 and it will be understood that it is placedin position after the coil 38 is positioned on the top of the plenum 31and before the inner cover 48 is dropped over the assembly. Other meansmay be provided for preventing the by-passing of atmosphere from theblower 25 past the trays 49, without getting through them, into the coil38. For example, a bafiie wall member could be carried by he inner cover48 and extend across between the Wall of the cover and the outerperiphery of the top tray 40.

As is necessary to purge the atmosphere within the inner cover 48 of theair at the start of a chromizing operation, an atmosphere outlet pipe 60extends up through the base 26 up into the space within the inner cover48 and it is provided with suitable valves and pressure regulatingdevices (not shown) to control the how therethrough. One or more gasinlet pipes 61 also extend through the base 20, and, as seen in FIGURE1, preferably have their upper outlet ends 62 disposed closely adjacentthe source of chromium.

In the operation of the apparatus described above, assuming that thetrays 40 are properly filled with a suitable source of chromium, theopen coil 38 is placed in position on the grid 35 of the plenum chamber31, the top baffle member 56 is put in place and the inner cover 48 isthen lowered over the entire assembly. Next the bell-type heat.- ingfurnace 54 is positioned over the inner cover 48 and the atmospherewithin-the inner cover 48 is purged of air by causing a suitable carriergas, for example a mixture of approximately nitrogen and 5% hydrogen tobe charged into the inner cover 48 through the pipe 61 while permittingthe escape of atmosphere through the outlet 60.

Before the purging operation, the blower 25 is started and then thefurnace 54 is fired up to proper temperature. Because of the arrangementof the battles and support members, the flow of the atmospherecirculated by the blower 25 is, as seen from the arrows of FIGURE 1,radially outwardly from theblower 25 past the diffuser vanes 24, outthrough the outlet passage-30 and upwardly along the wall of the innercover 48 where it is heated from the cover 48. Due to the inclinedbaflie spacers 41 between the trays 40, and to the top bafile member 56,all of the atmosphere must necessarily pass upwardly through theperforated bottom 44 of the trays 40 and through the source of chromium46 thereon. The screening 47 on top of the coating material 46 preventsthe particles of chromium from being blown out of the trays. It thenpasses up along the outer surface of the open coil 38 and, as it cannotpass down through the center opening in coil 38 because of the closureplate 37, moves downwardly through the spaces between the laps of thecoil 38 in intimate contact with all parts of the entire surface of thestrip material making up the coil. After the atmosphere leaves thebottom of open coil 38, it passes through the supporting grid 35 and isdirected by the inclined bafile wall .33 to the central opening 28 ofthe blower 25. This circulation is continued as long as the blower 25operates.

By use of the apparatus described, the circulating atmosphere may beutilized first to heat the charge to the desired temperature withoutcoating action and then, after admission of the activating agent, forconveying the metallic chromium from the granular material 46 to, anddistributing it uniformly and evenly over, the entire surface of thestripv metal making up the open coil 38.

The construction material of the retort in many cases may be plaincarbon steel or low alloy steel which will undergo chromizing in thefurnaces. There may be advantages, however, in certain cases inemploying heat resisting alloys such as high chromium and high chromiumnickel alloys including stainless steel and Inconel (79.5% nickel; 13%chromium; 6.5% iron; 0.25% manganese; 0.25% silicon; 0.2% copper).

Because of the importance of avoiding oxygen and moisture within theretort, it is very desirable that any fire brick or other refractoryused in construction of the retort be sheathed or covered with metal sothat it will not be exposed to the gases within the retort.

MOTION OF THE SOURCE OF CHROMIUM If desired, the bed formed of thesource of chromium may be in motion or fluidized, by desirably blowingthe gases of the retort through the bed at a suitable velocity andemploying a particle size which is of fine enough and sufiicient beddepth to provide levitation.

It will be understood that, of course, grain size control can be used sothat some of the particles of the source of ferrochrome are of largersize which will not levitate and others are of smaller size which willlevitate in the fluidized bed. Screen will suitably prevent theparticles from becoming entrained in the gases.

In view of our invention and disclosure, variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the art to obtain all or part of the benefits of ourinvention without copying the product shown, and we, therefore, claimall such insofar as they fall within the reasonable spirit and scope ofour claims.

In view of our invention and disclosure, what we claim as new and desireto secure by Letters Patent is:

1. A chromized ferrous metal open coil having a content of an alloy inthe metal of the class consisting of titanium, columbium, vanadium andtantalum of at least four times the carbon content, with the uncombinedcarbon content of the metal being not in excess of 0.03%, and producedby a process which comprises providing a retort which is closed withrespect to the atmosphere, placing in the retort ferrous metal work tobe chromized and closing the retort, introducing into the retort ahalogen acid gas, introducing into the retort a carrier gas free fromnitrogen contamination, maintaining the retort and the work at atemperature of between 1500 and 2300 F., providing a source of chromiumout of contact with the work, distributed in a series of layers throughwhich the gas can flow sequentially, there being a ratio of the totalsurface area of the source of chromium to the total surface area of thework which is between 0.8 to 1 and 5 to 1, maintaining the source ofchromium at a temperature between 1500 and 2300 F., providing a channelfrom the source of chromium to the work and providing a return channelfrom the work to the source of chromium, circulating the halogen gas andthe carrier gas in admixture under positive pressure sequentiallythrough the layers of the source of chromium and then in contact withthe work to chromize the work and then back through the source ofchromium to regenerate the gas, maintaining the chromizing potentialhigh by withdrawing from the retort increments of carrier gas andhalogen gas and maintaining a sufiiciently low dew point in the inletgases, so that the dew point in the exit gas is not in excess of +5 F.,and continuing the circulation of the gas and the chromizing for a timeof at least 5 hours.

2. An article of claim 1, produced by a process in which the carrier gasis oxygen-free hydrogen having a dew point when introduced into theretort of less than about F.

3. An article of claim 1, produced by a process in which the halogen gasis hydrogen bromide and in which the carrier gas comprises oxygen-freeand nitrogen-free hydrogen.

4. An article of claim 3, produced by a process in which the source ofchromium is porous, which comprises maintaining a percentage of hydrogenbromide by volume at 70 F. and one atmosphere pressure in relation tothe chromizing temperature which conforms with the following:

5. An article of claim 1, produced by a process which comprises holdingthe source of chromium containing iron at a temperature higher than thetemperature of the work prior to chromizing so as to diffuse chromium tothe surface and diffuse iron to the interior of the source of chromiumand thus replenish chromium at the surface.

6. A chromized ferrous metal article having a content of an alloy in themetal of the class consisting of titanium, columbium, vanadium andtantalum of at least four times the carbon content, with the uncombinedcarbon content of the metal being not in excess of 0.03%, and producedby a process of chromizing a coil of ferrous metal sheet having openspaces between laps, which comprises placing said coil in a chromizingretort with open spaces between the laps of the coil, maintaining in theretort a series of sequential layers of a source of chromium out ofcontact with the coil, establishing in the retort a circulating path forgas, which path brings the bulk of the gas within the retort intocontact with the surface of the coil between the laps and then passessuch gas sequentially through the layers of source of chromium, theratio of the total surface area of the source of chromium to the totalsurface area of the sheet being between 0.8 to 1 and 5 to 1, introducinginto the retort a halogen-acid gas, introducing into the retort acarrier gas free from nitrogen contamination, continuously circulatingsaid halogen gas and said carrier gas through said path between the lapsof the open coil and through the layers of source of chromium,maintaining the retort, the source of chromium, and the ferrous metalsheet at a temperature of between 1600" and 1800 F., withdrawingincrements of gas from the retort, the dew point of the inlet gas andthe rate of withdrawal being maintained so that the dew point of theexit gas is not in excess of +5 F., and continuing the chromizingoperation for a time of at least 5 hours.

7. An article of claim 6, produced by a process in which said halogen isbromine in the form of a material of the class consisting of hydrogenbromide, bromine and chromous bromide, and in which said carrier gascomprises hydrogen free from oxygen and free from nitrogen.

8. An article of claim 6, produced by a process in which the ratio ofsurface areas is between 3 to 1 and 5 to 1.

9. An article of claim 6, produced by a process in which the spacingbetween laps is in the range from 0.13 to 0.22 inch, inclusive.

(References on following page) 19 20 4 References Cited 2,836,513 5/1958 Samuel 117107.2 X 222128 81823 811:: ;;;;;;;;;-%7@z8?; Z1322 $2222259 11 9 6 52 5 FOREIGN PATENTS 4/1957 De LOI; 29--196X 685,683 1/1953Great Britain- 7/1957 Stareck 29-1966 X HYLAND BIZOT, Primary Examiner.

1. A CHROMIZED FERROUS METAL OPEN COIL HAVING A CONTENT OF AN ALLOY INTHE METAL OF THE CLASS CONSISTING OF TITANIUM, COLUMBIUM, VANADIUM ANDTANTALUM OF AT LEAST FOUR TIMES THE CARBON CONTENT, WITH THE UNCOMBINEDCARBON CONTENT OF THE METAL BEING NOT IN EXCESS OF 0.03%, AND PRODUCEDBY A PROCESS WHICH COMPRISES PROVIDING A RETORT WHICH IS CLOSED WITHRESPECT TO THE ATMOSPHERE, PLACING IN THE RETORT FERROUS METAL WORK TOBE CHROMIZED AND CLOSING THE RETORT, INTRODUCING INTO THE RETORT AHALOGEN ACID GAS, INTRODUCING INTO THE RETORT A CARRIER GAS FREE FROMNITROGEN CONTAMINATION, MAINTAINING THE RETORT AND THE WORK AT ATEMPERATURE OF BETWEEN 1500* AND 2300*F., PROVIDING A SOURCE OF CHROMIUMOUT OF CONTACT WITH THE WORK, DISTRIBUTED IN A SERIES OF LAYERS THROUGHWHICH THE GAS CAN FLOW SEQUENTIALLY, THERE BEING A RATIO OF THE TOTALSURFACE AREA OF THE SOURCE OF CHROMIUM TO THE TOTAL SURFACE AREA OF THEWORK WHICH IS BETWEEN 0.8 AND 1 AND